• like butta

    From Phil Hobbs@21:1/5 to All on Thu Jan 23 03:43:03 2025
    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of
    18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g. hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000
    gees, and over its lifetime it might see around 1E6 blows. Challenging.

    Doing <200 ps TDR on a balanced line obviously needs a very wideband balun. We’re using a plain ferrite design based on Ferroxcube 61 sleeves on 1.25mm coax. The resulting 50-Ω differential mode goes into a machined tapered structure that maintains 50 ohms while spreading out the mode to match the 25-mm tine spacing.

    For survivability, the whole thing is being potted in very hard epoxy with
    a dielectric constant of 3.5ish. This means that it’s hard to iterate—we get one try per apparatus.

    Sooooo, I tried making a soft material with that epsilon, which turns out
    to be nontrivial. My initial thought was to use alumina lapping powder
    (9.3) for filler and vaseline (2.0) for the matrix, but it turned into
    clumps by the time it got to 3.0.

    Casting about for alternatives, I remembered the butter in the lab fridge,
    so I measured that: 3.8, better than good enough.

    We were able to verify the design without doing anything irrevocable, and
    there was some left over for the bagels.

    Fun.

    Cheers

    Phil Hobbs
    --
    Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

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  • From bitrex@21:1/5 to Phil Hobbs on Wed Jan 22 23:21:36 2025
    On 1/22/2025 10:43 PM, Phil Hobbs wrote:
    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of
    18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g. hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000 gees, and over its lifetime it might see around 1E6 blows. Challenging.

    Doing <200 ps TDR on a balanced line obviously needs a very wideband balun. We’re using a plain ferrite design based on Ferroxcube 61 sleeves on 1.25mm coax. The resulting 50-Ω differential mode goes into a machined tapered structure that maintains 50 ohms while spreading out the mode to match the 25-mm tine spacing.

    For survivability, the whole thing is being potted in very hard epoxy with
    a dielectric constant of 3.5ish. This means that it’s hard to iterate—we get one try per apparatus.

    Sooooo, I tried making a soft material with that epsilon, which turns out
    to be nontrivial. My initial thought was to use alumina lapping powder
    (9.3) for filler and vaseline (2.0) for the matrix, but it turned into
    clumps by the time it got to 3.0.

    Casting about for alternatives, I remembered the butter in the lab fridge,
    so I measured that: 3.8, better than good enough.

    We were able to verify the design without doing anything irrevocable, and there was some left over for the bagels.

    Fun.

    Cheers

    Phil Hobbs



    Cold you please test ninja turtle ooze and report back. It's some kind
    of guar gum/vinyl acetate and borate gelatin:

    <https://en.wikipedia.org/wiki/Flubber_(material)>

    <https://www.amazon.com/Teenage-Mutant-Ninja-Turtles-83700CO/dp/B0C6RFTJF2/>

    --
    This email has been checked for viruses by AVG antivirus software.
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  • From john larkin@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Wed Jan 22 20:35:14 2025
    On Thu, 23 Jan 2025 03:43:03 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of
    18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g. >hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000 >gees, and over its lifetime it might see around 1E6 blows. Challenging.

    Doing <200 ps TDR on a balanced line obviously needs a very wideband balun. >We’re using a plain ferrite design based on Ferroxcube 61 sleeves on 1.25mm >coax. The resulting 50-? differential mode goes into a machined tapered >structure that maintains 50 ohms while spreading out the mode to match the >25-mm tine spacing.

    We've got excellent results with micro-coax on pot cores.

    https://www.dropbox.com/scl/fi/o2r1m53es9rb3e7nbsnsr/Pot_Core_TXline.JPG?rlkey=t1g3rp0erz72tqchb99fupfu7&raw=1

    https://www.dropbox.com/scl/fi/ie7gzmdwuw4gqgy1pxpjl/TX_1.jpg?rlkey=xllwjn2cg0a0t3yjh90om9ap1&raw=1


    For survivability, the whole thing is being potted in very hard epoxy with
    a dielectric constant of 3.5ish. This means that it’s hard to iterate—we
    get one try per apparatus.

    Sooooo, I tried making a soft material with that epsilon, which turns out
    to be nontrivial. My initial thought was to use alumina lapping powder
    (9.3) for filler and vaseline (2.0) for the matrix, but it turned into
    clumps by the time it got to 3.0.

    Casting about for alternatives, I remembered the butter in the lab fridge,
    so I measured that: 3.8, better than good enough.

    We were able to verify the design without doing anything irrevocable, and >there was some left over for the bagels.

    Fun.

    Cheers

    Phil Hobbs

    Are you doing a reverse convolution to beautify the step waveform? You
    can tolerate a really ugly TDR if you can make a FIR filter to pound
    it flat.

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  • From Phil Hobbs@21:1/5 to bitrex on Thu Jan 23 13:41:31 2025
    bitrex <user@example.net> wrote:
    On 1/22/2025 10:43 PM, Phil Hobbs wrote:
    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of
    18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g.
    hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000
    gees, and over its lifetime it might see around 1E6 blows. Challenging.

    Doing <200 ps TDR on a balanced line obviously needs a very wideband balun. >> We’re using a plain ferrite design based on Ferroxcube 61 sleeves on 1.25mm
    coax. The resulting 50-Ω differential mode goes into a machined tapered
    structure that maintains 50 ohms while spreading out the mode to match the >> 25-mm tine spacing.

    For survivability, the whole thing is being potted in very hard epoxy with >> a dielectric constant of 3.5ish. This means that it’s hard to iterate—we >> get one try per apparatus.

    Sooooo, I tried making a soft material with that epsilon, which turns out
    to be nontrivial. My initial thought was to use alumina lapping powder
    (9.3) for filler and vaseline (2.0) for the matrix, but it turned into
    clumps by the time it got to 3.0.

    Casting about for alternatives, I remembered the butter in the lab fridge, >> so I measured that: 3.8, better than good enough.

    We were able to verify the design without doing anything irrevocable, and
    there was some left over for the bagels.

    Fun.

    Cheers

    Phil Hobbs



    Cold you please test ninja turtle ooze and report back. It's some kind
    of guar gum/vinyl acetate and borate gelatin:

    <https://en.wikipedia.org/wiki/Flubber_(material)>

    <https://www.amazon.com/Teenage-Mutant-Ninja-Turtles-83700CO/dp/B0C6RFTJF2/>


    Nah, Western religious art, particularly Italian Renaissance stuff, leaves
    me cold. Cute little cupids representing the glorious cherubim, Madonnas modeled after artists’ paramours, corrupt prelates and donors painted into biblical scenes, blech.

    Cheers

    Phil Hobbs



    --
    Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

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  • From bitrex@21:1/5 to Phil Hobbs on Fri Jan 24 00:29:26 2025
    On 1/23/2025 8:41 AM, Phil Hobbs wrote:
    bitrex <user@example.net> wrote:
    On 1/22/2025 10:43 PM, Phil Hobbs wrote:
    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of
    18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g. >>> hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000 >>> gees, and over its lifetime it might see around 1E6 blows. Challenging.

    Doing <200 ps TDR on a balanced line obviously needs a very wideband balun. >>> We’re using a plain ferrite design based on Ferroxcube 61 sleeves on 1.25mm
    coax. The resulting 50-Ω differential mode goes into a machined tapered >>> structure that maintains 50 ohms while spreading out the mode to match the >>> 25-mm tine spacing.

    For survivability, the whole thing is being potted in very hard epoxy with >>> a dielectric constant of 3.5ish. This means that it’s hard to iterate—we
    get one try per apparatus.

    Sooooo, I tried making a soft material with that epsilon, which turns out >>> to be nontrivial. My initial thought was to use alumina lapping powder
    (9.3) for filler and vaseline (2.0) for the matrix, but it turned into
    clumps by the time it got to 3.0.

    Casting about for alternatives, I remembered the butter in the lab fridge, >>> so I measured that: 3.8, better than good enough.

    We were able to verify the design without doing anything irrevocable, and >>> there was some left over for the bagels.

    Fun.

    Cheers

    Phil Hobbs



    Cold you please test ninja turtle ooze and report back. It's some kind
    of guar gum/vinyl acetate and borate gelatin:

    <https://en.wikipedia.org/wiki/Flubber_(material)>

    <https://www.amazon.com/Teenage-Mutant-Ninja-Turtles-83700CO/dp/B0C6RFTJF2/> >>

    Nah, Western religious art, particularly Italian Renaissance stuff, leaves
    me cold. Cute little cupids representing the glorious cherubim, Madonnas modeled after artists’ paramours, corrupt prelates and donors painted into biblical scenes, blech.

    Cheers

    Phil Hobbs




    Right well I don't know if pizza's dielectric properties have been
    studied thoroughly but there's been a lot of work done on Jell-O, it
    seems pretty customizable. There's always room for Jell-O

    <https://pubs.acs.org/doi/pdf/10.1021/j150393a011>

    --
    This email has been checked for viruses by AVG antivirus software.
    www.avg.com

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  • From Glen Walpert@21:1/5 to Phil Hobbs on Sat Jan 25 01:03:30 2025
    On Thu, 23 Jan 2025 03:43:03 -0000 (UTC), Phil Hobbs wrote:

    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of
    18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g. hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000 gees, and over its lifetime it might see around 1E6 blows. Challenging.

    Have you considered using a mechanical low pass filter on your slide
    hammer, a pad on either the hammer or anvil striking surfaces? Rawhide
    faced iron hammers are very good at driving stakes into very hard soil
    without the severe impulse of a metal on metal impact, and while rawhide
    might not be good for 1E6 cycles you could try rubber sheet or heavy
    gasket material, tune the response by changing thickness and hardness to
    suit. Presumably the same slide hammer is used for removal, so you would
    want to pad the upper surface also.

    Between padding the hammer and compliant mounting of your circuit (two
    series low pass filters) you should be able to keep the acceleration of
    your circuit module to something reasonable.

    Hard epoxy potting compounds can put a lot of stress on parts from
    differential thermal expansion, sometimes a compliant layer of a more
    flexible material is used over sensitive components before potting with
    hard epoxy.

    Glen

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  • From Phil Hobbs@21:1/5 to Glen Walpert on Sat Jan 25 16:22:25 2025
    Glen Walpert <nospam@null.void> wrote:
    On Thu, 23 Jan 2025 03:43:03 -0000 (UTC), Phil Hobbs wrote:

    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of
    18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g.
    hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000
    gees, and over its lifetime it might see around 1E6 blows. Challenging.

    Have you considered using a mechanical low pass filter on your slide
    hammer, a pad on either the hammer or anvil striking surfaces? Rawhide
    faced iron hammers are very good at driving stakes into very hard soil without the severe impulse of a metal on metal impact, and while rawhide might not be good for 1E6 cycles you could try rubber sheet or heavy
    gasket material, tune the response by changing thickness and hardness to suit. Presumably the same slide hammer is used for removal, so you would want to pad the upper surface also.

    Between padding the hammer and compliant mounting of your circuit (two
    series low pass filters) you should be able to keep the acceleration of
    your circuit module to something reasonable.

    Hard epoxy potting compounds can put a lot of stress on parts from differential thermal expansion, sometimes a compliant layer of a more flexible material is used over sensitive components before potting with
    hard epoxy.

    Glen

    Thanks, that’s interesting.
    We’ve thought about a pad, but haven’t done any studies yet. The problem of hard epoxy ripping things apart is pretty well known, I think—as you say,
    the fix is a thin layer of RTV or something like that.

    I didn’t know about rawhide-faced hammers, which sound cool. With hard materials, the pulse width equals the length of the impactor divided by the speed of sound in the material, which in this case is just about 20us. The force is the change of momentum divided by the pulse width.

    If the pad compresses by 1 mm when the slide hammer arrives at 2 m/s, the
    pulse width is about 500us, so the peak acceleration would be more like
    1000 gees.

    Cheers

    Phil Hobbs


    --
    Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

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  • From =?UTF-8?B?Q2FybCBJamFtZXM=?=@21:1/5 to All on Sat Jan 25 18:44:56 2025
    On Thu Jan 23 03:43:03 2025 Phil Hobbs wrote:
    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of
    18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g. hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000 gees, and over its lifetime it might see around 1E6 blows. Challenging.

    Doing <200 ps TDR on a balanced line obviously needs a very wideband balun. We?re using a plain ferrite design based on Ferroxcube 61 sleeves on 1.25mm coax. The resulting 50-? differential mode goes into a machined tapered structure that maintains 50 ohms while spreading out the mode to match the 25-mm tine spacing.

    For survivability, the whole thing is being potted in very hard epoxy with
    a dielectric constant of 3.5ish. This means that it?s hard to iterate?we
    get one try per apparatus.

    Sooooo, I tried making a soft material with that epsilon, which turns out
    to be nontrivial. My initial thought was to use alumina lapping powder
    (9.3) for filler and vaseline (2.0) for the matrix, but it turned into
    clumps by the time it got to 3.0.

    Casting about for alternatives, I remembered the butter in the lab fridge,
    so I measured that: 3.8, better than good enough.

    We were able to verify the design without doing anything irrevocable, and there was some left over for the bagels.

    Fun.

    Cheers

    Phil Hobbs
    --
    Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

    Just for future reference if you want a material much more rigid than butter at room temperature you could try cocoa butter. If you make chocolates it is probably already in your pantry :-). It's a mix of triglycerides that melt between about 90 and
    100 F so melting it to cast in place or paint it on or clean it back off would be pretty easy. I found one online reference using Brave search that listed dielectric constants for some fatty acids and trglycerides similar to those in cocoa butter. They
    were between about 3 and 3.4 so right in the range you were lookng for. Anyway, I was curious and bored ...

    Regards,
    Carl

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  • From KevinJ93@21:1/5 to Phil Hobbs on Sat Jan 25 11:45:30 2025
    On 1/25/25 8:22 AM, Phil Hobbs wrote:
    Glen Walpert <nospam@null.void> wrote:
    On Thu, 23 Jan 2025 03:43:03 -0000 (UTC), Phil Hobbs wrote:

    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of
    18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g. >>> hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000 >>> gees, and over its lifetime it might see around 1E6 blows. Challenging.

    Have you considered using a mechanical low pass filter on your slide
    hammer, a pad on either the hammer or anvil striking surfaces? Rawhide
    faced iron hammers are very good at driving stakes into very hard soil
    without the severe impulse of a metal on metal impact, and while rawhide
    might not be good for 1E6 cycles you could try rubber sheet or heavy
    gasket material, tune the response by changing thickness and hardness to
    suit. Presumably the same slide hammer is used for removal, so you would
    want to pad the upper surface also.

    Between padding the hammer and compliant mounting of your circuit (two
    series low pass filters) you should be able to keep the acceleration of
    your circuit module to something reasonable.

    Hard epoxy potting compounds can put a lot of stress on parts from
    differential thermal expansion, sometimes a compliant layer of a more
    flexible material is used over sensitive components before potting with
    hard epoxy.

    Glen

    Thanks, that’s interesting.
    We’ve thought about a pad, but haven’t done any studies yet. The problem of
    hard epoxy ripping things apart is pretty well known, I think—as you say, the fix is a thin layer of RTV or something like that.

    I didn’t know about rawhide-faced hammers, which sound cool. With hard materials, the pulse width equals the length of the impactor divided by the speed of sound in the material, which in this case is just about 20us. The force is the change of momentum divided by the pulse width.

    If the pad compresses by 1 mm when the slide hammer arrives at 2 m/s, the pulse width is about 500us, so the peak acceleration would be more like
    1000 gees.

    Cheers

    Phil Hobbs



    There are also "Dead-Blow Hammers" used for automotive bodywork. They
    spread the strike over a longer period, often by being hollow and filled
    with steel shot.

    https://en.wikipedia.org/wiki/Dead-blow_hammer

    kw

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  • From Phil Hobbs@21:1/5 to kevin_es@whitedigs.com on Sun Jan 26 23:52:23 2025
    KevinJ93 <kevin_es@whitedigs.com> wrote:
    On 1/25/25 8:22 AM, Phil Hobbs wrote:
    Glen Walpert <nospam@null.void> wrote:
    On Thu, 23 Jan 2025 03:43:03 -0000 (UTC), Phil Hobbs wrote:

    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of >>>> 18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g. >>>> hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000 >>>> gees, and over its lifetime it might see around 1E6 blows. Challenging. >>>
    Have you considered using a mechanical low pass filter on your slide
    hammer, a pad on either the hammer or anvil striking surfaces? Rawhide
    faced iron hammers are very good at driving stakes into very hard soil
    without the severe impulse of a metal on metal impact, and while rawhide >>> might not be good for 1E6 cycles you could try rubber sheet or heavy
    gasket material, tune the response by changing thickness and hardness to >>> suit. Presumably the same slide hammer is used for removal, so you would >>> want to pad the upper surface also.

    Between padding the hammer and compliant mounting of your circuit (two
    series low pass filters) you should be able to keep the acceleration of
    your circuit module to something reasonable.

    Hard epoxy potting compounds can put a lot of stress on parts from
    differential thermal expansion, sometimes a compliant layer of a more
    flexible material is used over sensitive components before potting with
    hard epoxy.

    Glen

    Thanks, that’s interesting.
    We’ve thought about a pad, but haven’t done any studies yet. The problem of
    hard epoxy ripping things apart is pretty well known, I think—as you say, >> the fix is a thin layer of RTV or something like that.

    I didn’t know about rawhide-faced hammers, which sound cool. With hard
    materials, the pulse width equals the length of the impactor divided by the >> speed of sound in the material, which in this case is just about 20us. The >> force is the change of momentum divided by the pulse width.

    If the pad compresses by 1 mm when the slide hammer arrives at 2 m/s, the
    pulse width is about 500us, so the peak acceleration would be more like
    1000 gees.

    Cheers

    Phil Hobbs



    There are also "Dead-Blow Hammers" used for automotive bodywork. They
    spread the strike over a longer period, often by being hollow and filled
    with steel shot.

    https://en.wikipedia.org/wiki/Dead-blow_hammer

    kw


    I had one of those back at IBM. They seem a bit on the wimpy side for
    pounding stakes into hardpan, no?

    Thanks

    Phil Hobbs

    --
    Dr Philip C D Hobbs Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics,
    Electro-optics, Photonics, Analog Electronics

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to Carl Ijames on Sun Jan 26 23:52:29 2025
    Carl Ijames <carl.ijamesXX@verizon.netYY> wrote:
    On Thu Jan 23 03:43:03 2025 Phil Hobbs wrote:
    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of
    18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g.
    hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000
    gees, and over its lifetime it might see around 1E6 blows. Challenging.

    Doing <200 ps TDR on a balanced line obviously needs a very wideband balun. >> We?re using a plain ferrite design based on Ferroxcube 61 sleeves on 1.25mm >> coax. The resulting 50-? differential mode goes into a machined tapered
    structure that maintains 50 ohms while spreading out the mode to match the >> 25-mm tine spacing.

    For survivability, the whole thing is being potted in very hard epoxy with >> a dielectric constant of 3.5ish. This means that it?s hard to iterate?we
    get one try per apparatus.

    Sooooo, I tried making a soft material with that epsilon, which turns out
    to be nontrivial. My initial thought was to use alumina lapping powder
    (9.3) for filler and vaseline (2.0) for the matrix, but it turned into
    clumps by the time it got to 3.0.

    Casting about for alternatives, I remembered the butter in the lab fridge, >> so I measured that: 3.8, better than good enough.

    We were able to verify the design without doing anything irrevocable, and
    there was some left over for the bagels.

    Fun.

    Cheers

    Phil Hobbs
    --
    Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / >> Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

    Just for future reference if you want a material much more rigid than
    butter at room temperature you could try cocoa butter. If you make chocolates it is probably already in your pantry :-). It's a mix of triglycerides that melt between about 90 and 100 F so melting it to cast
    in place or paint it on or clean it back off would be pretty easy. I
    found one online reference using Brave search that listed dielectric constants for some fatty acids and trglycerides similar to those in cocoa butter. They were between about 3 and 3.4 so right in the range you were lookng for. Anyway, I was curious and bored ...

    Regards,
    Carl

    White chocolate is clearly the future of RF design. ;)

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics,
    Electro-optics, Photonics, Analog Electronics

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to bitrex on Sun Jan 26 23:54:45 2025
    bitrex <user@example.net> wrote:
    On 1/23/2025 8:41 AM, Phil Hobbs wrote:
    bitrex <user@example.net> wrote:
    On 1/22/2025 10:43 PM, Phil Hobbs wrote:
    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of >>>> 18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g. >>>> hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000 >>>> gees, and over its lifetime it might see around 1E6 blows. Challenging. >>>>
    Doing <200 ps TDR on a balanced line obviously needs a very wideband balun.
    We’re using a plain ferrite design based on Ferroxcube 61 sleeves on 1.25mm
    coax. The resulting 50-Ω differential mode goes into a machined tapered >>>> structure that maintains 50 ohms while spreading out the mode to match the >>>> 25-mm tine spacing.

    For survivability, the whole thing is being potted in very hard epoxy with >>>> a dielectric constant of 3.5ish. This means that it’s hard to iterate—we
    get one try per apparatus.

    Sooooo, I tried making a soft material with that epsilon, which turns out >>>> to be nontrivial. My initial thought was to use alumina lapping powder >>>> (9.3) for filler and vaseline (2.0) for the matrix, but it turned into >>>> clumps by the time it got to 3.0.

    Casting about for alternatives, I remembered the butter in the lab fridge, >>>> so I measured that: 3.8, better than good enough.

    We were able to verify the design without doing anything irrevocable, and >>>> there was some left over for the bagels.

    Fun.

    Cheers

    Phil Hobbs



    Cold you please test ninja turtle ooze and report back. It's some kind
    of guar gum/vinyl acetate and borate gelatin:

    <https://en.wikipedia.org/wiki/Flubber_(material)>

    <https://www.amazon.com/Teenage-Mutant-Ninja-Turtles-83700CO/dp/B0C6RFTJF2/>


    Nah, Western religious art, particularly Italian Renaissance stuff, leaves >> me cold. Cute little cupids representing the glorious cherubim, Madonnas
    modeled after artists’ paramours, corrupt prelates and donors painted into >> biblical scenes, blech.

    Cheers

    Phil Hobbs




    Right well I don't know if pizza's dielectric properties have been
    studied thoroughly but there's been a lot of work done on Jell-O, it
    seems pretty customizable. There's always room for Jell-O

    <https://pubs.acs.org/doi/pdf/10.1021/j150393a011>


    It’s been used for making edible lasers.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics,
    Electro-optics, Photonics, Analog Electronics

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  • From Phil Hobbs@21:1/5 to john larkin on Mon Jan 27 00:15:30 2025
    john larkin <JL@gct.com> wrote:
    On Thu, 23 Jan 2025 03:43:03 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of
    18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g.
    hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000
    gees, and over its lifetime it might see around 1E6 blows. Challenging.

    Doing <200 ps TDR on a balanced line obviously needs a very wideband balun. >> WeÂ’re using a plain ferrite design based on Ferroxcube 61 sleeves on 1.25mm >> coax. The resulting 50-? differential mode goes into a machined tapered
    structure that maintains 50 ohms while spreading out the mode to match the >> 25-mm tine spacing.

    We've got excellent results with micro-coax on pot cores.

    https://www.dropbox.com/scl/fi/o2r1m53es9rb3e7nbsnsr/Pot_Core_TXline.JPG?rlkey=t1g3rp0erz72tqchb99fupfu7&raw=1

    https://www.dropbox.com/scl/fi/ie7gzmdwuw4gqgy1pxpjl/TX_1.jpg?rlkey=xllwjn2cg0a0t3yjh90om9ap1&raw=1

    I have that one in my tricks file, for sure.

    We’re using two ferrite sleeves as a 1:1 balun, which is the sideways version.


    For survivability, the whole thing is being potted in very hard epoxy with >> a dielectric constant of 3.5ish. This means that it’s hard to iterate—we >> get one try per apparatus.

    Sooooo, I tried making a soft material with that epsilon, which turns out
    to be nontrivial. My initial thought was to use alumina lapping powder
    (9.3) for filler and vaseline (2.0) for the matrix, but it turned into
    clumps by the time it got to 3.0.

    Casting about for alternatives, I remembered the butter in the lab fridge, >> so I measured that: 3.8, better than good enough.

    We were able to verify the design without doing anything irrevocable, and
    there was some left over for the bagels.

    Fun.

    Cheers

    Phil Hobbs

    Are you doing a reverse convolution to beautify the step waveform? You
    can tolerate a really ugly TDR if you can make a FIR filter to pound
    it flat.

    The step is actually very nice-looking—clean edges, flat pulse tops, no worries. We did have to make a couple of layout adjustments to get there,
    but the result shows TDR edges surprisingly similar to the SD-24’s.

    Ours is much less fancy, and needs a few pulses per delay value to get good convergence in the sampling loop, but in a not-too-scientific comparison on
    a 24-inch RG-188 cable, both the SD-24 and our gizmo showed 60-ps TDR edges (10-90%). Of course the SD-24 is more like 30 ps on a less lossy cable,
    but for $16 per completed board, including MCU and data converters, I’m pretty happy with it.

    Cheers

    Phil Hobbs





    --
    Dr Philip C D Hobbs Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics,
    Electro-optics, Photonics, Analog Electronics

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  • From Joe Gwinn@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Sun Jan 26 20:23:02 2025
    On Sun, 26 Jan 2025 23:52:23 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    KevinJ93 <kevin_es@whitedigs.com> wrote:
    On 1/25/25 8:22 AM, Phil Hobbs wrote:
    Glen Walpert <nospam@null.void> wrote:
    On Thu, 23 Jan 2025 03:43:03 -0000 (UTC), Phil Hobbs wrote:

    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of >>>>> 18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g. >>>>> hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000 >>>>> gees, and over its lifetime it might see around 1E6 blows. Challenging. >>>>
    Have you considered using a mechanical low pass filter on your slide
    hammer, a pad on either the hammer or anvil striking surfaces? Rawhide >>>> faced iron hammers are very good at driving stakes into very hard soil >>>> without the severe impulse of a metal on metal impact, and while rawhide >>>> might not be good for 1E6 cycles you could try rubber sheet or heavy
    gasket material, tune the response by changing thickness and hardness to >>>> suit. Presumably the same slide hammer is used for removal, so you would >>>> want to pad the upper surface also.

    Between padding the hammer and compliant mounting of your circuit (two >>>> series low pass filters) you should be able to keep the acceleration of >>>> your circuit module to something reasonable.

    Hard epoxy potting compounds can put a lot of stress on parts from
    differential thermal expansion, sometimes a compliant layer of a more
    flexible material is used over sensitive components before potting with >>>> hard epoxy.

    Glen

    Thanks, that’s interesting.
    We’ve thought about a pad, but haven’t done any studies yet. The problem of >>> hard epoxy ripping things apart is pretty well known, I think—as you say, >>> the fix is a thin layer of RTV or something like that.

    I didn’t know about rawhide-faced hammers, which sound cool.

    Rawhide faced iron hammers would certainly work.

    With hard
    materials, the pulse width equals the length of the impactor divided by the >>> speed of sound in the material, which in this case is just about 20us. The >>> force is the change of momentum divided by the pulse width.

    If the pad compresses by 1 mm when the slide hammer arrives at 2 m/s, the >>> pulse width is about 500us, so the peak acceleration would be more like
    1000 gees.

    Cheers

    Phil Hobbs



    There are also "Dead-Blow Hammers" used for automotive bodywork. They
    spread the strike over a longer period, often by being hollow and filled
    with steel shot.

    < https://en.wikipedia.org/wiki/Dead-blow_hammer>

    These are too soft to drive a steel rod.


    kw


    I had one of those back at IBM. They seem a bit on the wimpy side for >pounding stakes into hardpan, no?

    Not to worry, help is coming ...

    .<https://youtu.be/eIw3uNCDR_s>

    .<https://youtu.be/XWv2EvzwycM>


    There are smaller options.

    .<https://rohrermfg.com/brands/Man-Saver-Post-Driver.html>

    Joe

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  • From Chris Jones@21:1/5 to Phil Hobbs on Sat Feb 1 14:19:36 2025
    On 23/01/2025 2:43 pm, Phil Hobbs wrote:
    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of
    18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g. hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000 gees, and over its lifetime it might see around 1E6 blows. Challenging.

    Doing <200 ps TDR on a balanced line obviously needs a very wideband balun. We’re using a plain ferrite design based on Ferroxcube 61 sleeves on 1.25mm coax. The resulting 50-Ω differential mode goes into a machined tapered structure that maintains 50 ohms while spreading out the mode to match the 25-mm tine spacing.

    For survivability, the whole thing is being potted in very hard epoxy with
    a dielectric constant of 3.5ish. This means that it’s hard to iterate—we get one try per apparatus.

    Sooooo, I tried making a soft material with that epsilon, which turns out
    to be nontrivial. My initial thought was to use alumina lapping powder
    (9.3) for filler and vaseline (2.0) for the matrix, but it turned into
    clumps by the time it got to 3.0.

    Casting about for alternatives, I remembered the butter in the lab fridge,
    so I measured that: 3.8, better than good enough.

    We were able to verify the design without doing anything irrevocable, and there was some left over for the bagels.

    Fun.

    Cheers

    Phil Hobbs

    Butter seems to be pretty lossy at RF, but I don't know whether that
    matters.

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  • From Phil Hobbs@21:1/5 to Chris Jones on Sat Feb 1 03:27:43 2025
    Chris Jones <lugnut808@spam.yahoo.com> wrote:
    On 23/01/2025 2:43 pm, Phil Hobbs wrote:
    Simon and I are working on a TDR based soil moisture sensor for
    agriculture. Like many such things, it uses two parallel tines made of
    18/8 stainless, that form a balanced transmission line.

    Ours has a slide hammer for pounding it into really difficult soil, e.g.
    hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000
    gees, and over its lifetime it might see around 1E6 blows. Challenging.

    Doing <200 ps TDR on a balanced line obviously needs a very wideband balun. >> We’re using a plain ferrite design based on Ferroxcube 61 sleeves on 1.25mm
    coax. The resulting 50-Ω differential mode goes into a machined tapered
    structure that maintains 50 ohms while spreading out the mode to match the >> 25-mm tine spacing.

    For survivability, the whole thing is being potted in very hard epoxy with >> a dielectric constant of 3.5ish. This means that it’s hard to iterate—we >> get one try per apparatus.

    Sooooo, I tried making a soft material with that epsilon, which turns out
    to be nontrivial. My initial thought was to use alumina lapping powder
    (9.3) for filler and vaseline (2.0) for the matrix, but it turned into
    clumps by the time it got to 3.0.

    Casting about for alternatives, I remembered the butter in the lab fridge, >> so I measured that: 3.8, better than good enough.

    We were able to verify the design without doing anything irrevocable, and
    there was some left over for the bagels.

    Fun.

    Cheers

    Phil Hobbs

    Butter seems to be pretty lossy at RF, but I don't know whether that
    matters.



    It was only an inch or two of propagation distance, and we were mostly interested in the performance of the mode-matching structures.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

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